1. Field of the Invention
The present invention relates to a hydrogen permeable alloy as a hydrogen permeable membrane for use in the separation and purification of hydrogen.
2. Description of the Related Art
Highly pure hydrogen has been used to produce semiconductors, optical fibers and chemicals. The amount of highly pure hydrogen in use has been increasing every year. In recent years, hydrogen has also become known as a fuel for fuel cells. If fuel cells are used on a large scale in the future, highly pure hydrogen will be needed in large amounts. For this reason, it is desirable to develop a method capable of mass-producing hydrogen, including (1) a water electrolysis method involving the use of non-fossil fuel, and (2) a steam reforming method of hydrocarbon involving the use of fossil fuel. In regards to the electrolysis method (1), water electrolysis generation as an electric supply has been under study, but it is difficult to put it into practical use at the present technical level. Accordingly, at present it is most realistic to produce hydrogen by steam reforming hydrocarbons (2).
When producing hydrogen by stream reforming of hydrocarbons, the reaction system contains impure gases such as CO, CO2, H2O and CH4 in addition to a large amount of hydrogen. In order to utilize hydrogen as a raw material to be supplied to the fuel cell, hydrogen must be separated and purified from these impurities. Further, Pt electrodes in the fuel cell will undergo damage unless the content of CO in purified hydrogen is reduced to 10 ppm or less. In other words, in order to use hydrogen in the fuel cell, hydrogen must be purified to a high degree.
Examples of hydrogen purifying methods include the absorption method, cryogenic distillation method, and the membrane separation method. Among these, the most efficient method for producing highly pure hydrogen is the membrane separation method utilizing metals.
The mechanism of the permeation of hydrogen in the metallic membrane is described below. When a hydrogen pressure difference occurs across the metallic membrane, hydrogen molecules (H2) are dissociated into hydrogen atoms (H) on the surface of the high pressure side of the metallic membrane. The hydrogen atoms are then dissolved into the metal. These hydrogen atoms permeate through the metallic membrane to the low pressure side, on which they are then combined to produce H2 molecules which then come out of the metallic membrane. This results in the purification of hydrogen. The purification of hydrogen through a metallic membrane is characterized by an extremely great separation factor and permeability. The purification of hydrogen using a metallic membrane allows the purity of hydrogen to rise from about 99% to about 99.99999%. Accordingly, it can be said that the membrane separation method using a metallic membrane is suitable for the purification of hydrogen in order to produce highly pure hydrogen for fuel cells.
In regards to the hydrogen permeable membrane technique, the Pd alloy has been mainly put into practical use. However, when fuel cells are used on a large scale, a large amount of hydrogen will be needed. Accordingly, the demand for the Pd—Ag alloy as a hydrogen permeable metallic membrane will grow. If this happens, Pd, which is an expensive and scarce resource, will be the limiting factor that makes it impossible for the Pd alloy membrane to meet the industrial demand. Therefore, it is keenly desirable to develop substitute materials for the metallic membrane.
For example, JP-A-11-276866 discloses an alloy based on V, Nb or Ta. V, Nb and Ta are known to have excellent hydrogen permeability as compared with the Pd alloy. However, these elements have an extremely great hydrogen solubility and thus can easily undergo cracking due to hydrogen embrittlement when used in a simple substance. Therefore, it is necessary for these elements to be alloyed to have a reduced hydrogen solubility. In general, however, these elements exhibit deteriorated hydrogen permeability when they have a cracking resistance-enhancing element incorporated therein. JP-A-11-276866 makes no definite reference to the kind of additive elements and their use and thus cannot provide practical hydrogen permeable alloys excellent both in hydrogen permeability and cracking resistance.
In addition, JP-A-2000-159503 also discloses Nb-based hydrogen permeable alloys. In JP-A-2000-159503, it is assumed that these alloys occur in a single phase. However, it is difficult to cause a single phase to attain conflicting properties, i.e., hydrogen permeability and hydrogen embrittlement resistance. In order to attempt to inhibit the hydrogen embrittlement of these alloys, the hydrogen solubility of these alloys must be unavoidably lowered, causing the deterioration of hydrogen permeability.
As a means of inhibiting hydrogen embrittlement, JP-A-2004-42017 discloses a hydrogen permeable membrane made of an amorphous alloy. However, since the diffusion coefficient of hydrogen in an amorphous alloy is generally lower than that of crystalline materials, the proposed hydrogen permeable membrane cannot provide high hydrogen permeability. Further, since such an amorphous material undergoes crystallization when the temperature rises, the working temperature is limited. In particular, an amorphous alloy prepared for hydrogen permeation contains elements having a high bonding force to hydrogen and thus undergoes crystallization at lower temperatures in hydrogen.
In order to render a hydrogen permeable alloy excellent both in hydrogen permeability and hydrogen embrittlement resistance, the idea of a composite alloy has been proposed which causes different phases to attain hydrogen permeability and hydrogen embrittlement resistance. In this light, some of the present inventors propose an Nb—Ti—Co-based alloy. This alloy causes the (Nb, Ti) phase and the CoTi phase to attain hydrogen permeability and hydrogen embrittlement resistance, respectively, making it possible to attain hydrogen permeability and hydrogen embrittlement resistance which are equal to or better than that of Pd alloy membranes.
However, the related Nb—Ti—Co alloy leaves something to be desired in hydrogen permeability and thus needs to be improved in that aspect. In order to put the Nb—Ti—Co alloy into practical use, it is necessary to reduce the thickness of the Nb—Ti—Co alloy to scores of micrometers to form a foil. The method favorable for reducing the thickness of Nb—Ti—Co alloy is a method involving the repetition of cold rolling and annealing. However, it is unknown how the structural change caused by this method affects the hydrogen permeability.